Na_V1.5 sodium channel window currents contribute to spontaneous firing in olfactory sensory neurons

Abstract

Frenz CT, Hansen A, Dupuis ND, Shultz N, Levinson SR, Finger TE, Dionne VE. Na V 1.5 sodium channel window currents contribute to spontaneous firing in olfactory sensory neurons. J Neurophysiol 112: 1091-1104. First published May 28, 2014 doi:10.1152/jn.00154.2014.-Olfactory sensory neurons (OSNs) fire spontaneously as well as in response to odor; both forms of firing are physiologically important. We studied voltage-gated Na ϩ channels in OSNs to assess their role in spontaneous activity. Whole cell patchclamp recordings from OSNs demonstrated both tetrodotoxin-sensitive and tetrodotoxin-resistant components of Na ϩ current. RT-PCR showed mRNAs for five of the nine different Na ϩ channel ␣-subunits in olfactory tissue; only one was tetrodotoxin resistant, the so-called cardiac subtype Na V 1.5. Immunohistochemical analysis indicated that Na V 1.5 is present in the apical knob of OSN dendrites but not in the axon. The Na V 1.5 channels in OSNs exhibited two important features: 1) a half-inactivation potential near Ϫ100 mV, well below the resting potential, and 2) a window current centered near the resting potential. The negative half-inactivation potential renders most Na V 1.5 channels in OSNs inactivated at the resting potential, while the window current indicates that the minor fraction of noninactivated Na V 1.5 channels have a small probability of opening spontaneously at the resting potential. When the tetrodotoxin-sensitive Na ϩ channels were blocked by nanomolar tetrodotoxin at the resting potential, spontaneous firing was suppressed as expected. Furthermore, selectively blocking Na V 1.5 channels with Zn 2ϩ in the absence of tetrodotoxin also suppressed spontaneous firing, indicating that Na V 1.5 channels are required for spontaneous activity despite resting inactivation. We propose that window currents produced by noninactivated Na V 1.5 channels are one source of the generator potentials that trigger spontaneous firing, while the upstroke and propagation of action potentials in OSNs are borne by the tetrodotoxin-sensitive Na ϩ channel subtypes.

Organization and Plasticity of Sodium Channel Expression in the Mouse Olfactory and Vomeronasal Epithelia

“…These cells can be identified by their specific morphology including a club-shaped soma that is positioned in the most apical MOE layer, extending a thick process towards the basal membrane (Elsaesser and Paysan, 2005). Consistent with a previous report (Frenz et al, 2014), we also noticed a subset of MOE microvillar cells that were labeled for Na v 1.7 (Figures 2B,D). We observed Na v 1.6 immunoreactivity in the apical cytosol of sustentacular cells, a non-neuronal cell type of the MOE with glia-like supportive function (Farbman, 1992).…”

“…Using real-time quantitative PCR, we identified six different Na v channel mRNAs in both MOE and VNO, demonstrating that these tissues share the same channels. Our results in the MOE are supported by previous reports that have identified these isoforms by RT-PCR (Weiss et al, 2011; Frenz et al, 2014) and deep RNA sequencing (Ibarra-Soria et al, 2014). In the VNO, Fieni et al (2003) identified mRNAs encoding Na v 1.1, Na v 1.2, and Na v 1.3.…”

“…To verify that the Na v isoforms we identified by qPCR localize to OSNs, we performed immunohistochemistry for the isoforms Na v 1.2, Na v 1.3, Na v 1.6 and Na v 1.7 in the MOE of adult mice (Figure 2) using previously established antibodies (Sage et al, 2007; Gao et al, 2009; Weiss et al, 2011). We did not investigate the distribution of Na v 1.5 in OSNs as this has already been reported by Frenz et al (2014). Olfactory tissue sections from B6 mice or from heterozygous OMP-GFP mice were subjected to immunohistochemistry using tyramid signal amplification.…”

“…Consistent with the cellular and behavioral phenotypes of mice harboring a conditional knockout mutation in Scn9a (Weiss et al, 2011), several additional investigations have provided evidence that Na v 1.7 is not the only sodium channel expressed in the peripheral olfactory system, but that other isoforms are also present and could perform specific roles in excitation. In the MOE, several different Na v isoforms have been identified in mouse OSNs by expression profiling (Sammeta et al, 2007), RT-PCR (Ahn et al, 2011; Weiss et al, 2011; Frenz et al, 2014), and deep RNA sequencing (Ibarra-Soria et al, 2014). Only recently, Na v 1.7, Na v 1.3 (Weiss et al, 2011), and Na v 1.5 (Frenz et al, 2014) were identified in mouse OSNs in addition to Na v 1.7 in rat OSNs (Ahn et al, 2011) using immunohistochemistry.…”

“…In the MOE, several different Na v isoforms have been identified in mouse OSNs by expression profiling (Sammeta et al, 2007), RT-PCR (Ahn et al, 2011; Weiss et al, 2011; Frenz et al, 2014), and deep RNA sequencing (Ibarra-Soria et al, 2014). Only recently, Na v 1.7, Na v 1.3 (Weiss et al, 2011), and Na v 1.5 (Frenz et al, 2014) were identified in mouse OSNs in addition to Na v 1.7 in rat OSNs (Ahn et al, 2011) using immunohistochemistry. In the vomeronasal organ (VNO), very little information is available on Na v channel expression in vomeronasal sensory neurons (VSNs).…”